1. Field of this Invention
This invention relates to a process for the production of 2-halopyridine derivatives having the formula: ##STR3## where X is halogen, R is --COOH or a lower alkyl radical and n is a number of 0 to 4.
2. Prior Art
2-Halopyridine compounds are accessible to numerous secondary reactions, since the halogen in the 2-position is relatively easily exchanged. For example, 2-chloro-5-pyridine carboxylic acid (6-chloronicotinic acid) serves as starting material for the production of 2-amino-5-pyridine carboxylic acid ester and amide (Chem. Abst., 90, 174689 e; Chem Abst., 88, 141704 m). A secondary product of 2-chloropyridine is pyridine-2-thiol-1-oxide (Omadine), an agent used against scales of the skin of the head. Furthermore, the 2-chloropyridine is an important intermediate product for numerous medicaments, for example, for antihistamines of the pheniramine-type. It is known to produce 2-chloropyridine derivatives, for example, 2-chloro-5-pyridine carboxylic acid (6-chloronicotinic acid), using PCl.sub.5 POCl.sub.3, whereby hydroxynicotinic acid is produced, for example, from malic acid. The process is complicated and because of the use of phosphorus chloride compounds is accompanied by considerable effluent problems.
2-Chloropyridine is produced according to known processes by the gaseous phase chlorination of pyridine in the presence of water (U.S. Pat. No. 3,920,657) or carbon tetrachloride (U.S. Pat. No. 3,153,044) at temperatures around 350.degree. C. The pyridine conversions are only around 34 to 50 percent. Unreacted pyridine is obtained as the hydrochloride and must be processed prior to reuse. Besides, other chlorination products also resulted which must be separated from the desired 2-chloropyridine.